Oscillator-limiter-detector apparatus and method



Oct. 13, 1964 w. D. KNIGHT 3,153,201

OSCILLATOR-LIMITER-DETECTOR APPARATUS AND METHOD Filed Dec. 5, 1961 8RFC 6 7| 24 T 5 u fi I INVENTOR. WALTER D. KNIGHT A T TOPNEV UnitedStates Patent Ofifice.

j 3,153,201 Patented Get-1 1964 oscuirnrou-rnnrunuurncron APPARATUS ANDMETHQD Waiter D. Knight, Berkeley, Calif., assignor, by mesneassignments, to the United States of America as repre-' sented by theSecretary of the Navy Filed Dec. 5, 1961, Ser. No. 157,285 2 (Ilairns.(Cl. 329 -150) Pound and W. D. Knight, Review of Scientific Instruments,volume 21, page 219 (1950), permits of sensitive operation with goodsignal-to-noise ratio only in the range of RF levels between 0.01 and 1volt. Schemes have been contrived which extend the range to microvoltlevels, using the technique of increasingv the gain of the RF amplifier.It has been shownthat reliable operation can be achieved at low levelsby incorporating a limiter in the feedback network. This, along withsuitably high RF amplification, facilitates stableoperation at lowvoltage levels,

An object of the present invention is to provide a modification of thecathode-coupled circuit which possesses someof the advantages of boththe marginal and the limited modes of operation, and to provide a methodfor operating such a circuit.

Other objects and many of the attendant advantages of V this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawing wherein the single figure is a;

circuit diagram illustrating a preferred embodiment of the circuit'ofthis invention. 7

The illustrated preferred embodiment of the circuit of this inventioncomprises a pair of multielectrode electron tube sections. These can beseparate vacuum tubes but preferably form sections of a multi-unit tube,shown as a twin triode but conceivably a triode-pentode or othercombination. By Way of example the figure shows a 6BQ7A and the valuesof the circuit parameters are appropriate for this tube. The first half2 of the tube is a cathode follower to which is connected the tankcircuit including coil L and condenser C. The second half 4 of the tubeoperates both as a cathode-driven detector (which provides a smalldegree of automatic level control), and as a plate-limited RF amplifier.Regeneration is provided by the capacitor 6.

The amplifier plate circuit is returned to a variable direct voltagesource, illustrated as a potentiometer 8. This plate supply voltagesource is constructed so as to supply a plate voltage to the amplifier,between plate and cathode, which never exceeds about one half of thenormal operating plate voltage for which the particular tube isdesigned. The desired limiting action of the apparatus is achieved byimposing this low plate voltage. The signal developed across the cathoderesistor 10 of the first section of the triode is cathode-coupled to thesection 4 of the triode through capacitor 12. The signal appearing atthe cathode 14 is amplified and limited at the plate 16' and fed back tothe input grid 18 via capacitor 6.

is the detector filter.

and capacitor 34 forms an RF filter to keep RF voltage I The plate load20 for tube section 4 is shown as an RF choke, typically 1 millihenry,which'is preferable, although a resistance could be substituted for it"of say l to 10 kilo-ohms. It is across this load 20 that there isdeveloped the regenerative voltage fed back through impedance 6 to grid18. Impedance 6 should be at least as high and preferablyhigher than theLC impedance.

The cathode load 22101 tube section 4 is shown as an RF choke, typically1 millihenry, but could be a resistance although this would not work aswell.

At junction 24 appears, for example, +50 volts D.C.,

while at junction 26 appears ground which is 0 volts D.C.

Hence 6 must be a D.C. blocking device such'as a condenser as shown ofsmall capacitance or aresistor plus a blocking condenser.

The network including resistor 28 and capacitor 30 The network includingresistor 32 out of the output appearing at 36, which typically may be anaudio voltage superimposed on the rectified RF voltage. The 33-oh-mresistors shown in the plate circuit and grid circuit of tube section 2are parasitic suppressors.

Operation The operation of the apparatus in its especially usefulfunction in nuclear magnetic resonance studies-is as follows. The coil Lof the tank circuit surrounds'a sample of material whose nuclearmagnetic resonance is to be studied. The oscillation of the devicestartsby perhaps random noise. A D.C. voltmeter or like instrument isplaced to measure the output, namely the voltage between output 36 andground. The potentiometer 8 is vacuum tube voltmeter is removed from thecircuit.

Usuallythe sample of material to be examined is immersed in a magneticfield, which field is varied at an audio rate. As the magnetic field isVaried and a point is reached at which the sample exhibits nuclearmagnetic resonance, the 'Q of L diminishes because of absorption ofenergy by the sample. As the Q goes down, the voltage across L goes downand hence the output at 36 decreases in proportion. It may be noted thatwhen the magnetic field surrounding'the sample is varied continuously atan audio rate, there appears at the output terminal 36 an audio outputsuperimposed on the direct voltage at 36. This output is what ismeasured in conducting the nuclear magnetic resonance studies and can beamplified with a tuned amplifier to reduce noise if desired.

This circuit is particularly suitable for application to problems innuclear resonance which require RF levels between '1 and 20 volts R.M.S.Near one volt the operation is marginal; at higher levels the limitingaction is a predominant. The arrangement shown in the drawing has beenused between 1 and 10 me. andat levels between 1 and 10 volts R.M.S.,with sensitivity equal to the best attainable with the usual marginalcircuit.

Since there are no time constants in the audio-frequency range, very lowmodulation frequencies can be used, and motorboating does not occur.Additionally, it may be noted that this circuit easily works tofrequencies of 50 me. or above, since a separate RF amplifier is notrequired, and since the feedback network is simple.

The method of the invention comprises operating the circuit of theinvention by supplying a plate supply voltage to the amplifier tubesection of such a magnitude that a the-potential difference between theplate and the cathode, is always maintained at less than one half of thenormal operating plate voltage for the particular tube being used.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. :It is therefore to beunderstood that within j=the scope :of the appended claims the inventionmay :be

practiced otherwise than as specifically described.

What is claimed is:

1. An oscillator-limiter-detector comprising a first electron tubesection serving'as a cathode follower; a tank circuit connected betweenground and the grid of said cathode follower to provide the input tosaid cathode tolplifie-r remains always at less than one half the normaloperating volt-age for said amplifier tube section; means connectedbetween said input tank circuit and the plate of said amplifier tubesection to feed back RF signal from said .plate to said input tankcircuit; and means connected between ground and grid in said amplifiertube section to provide a filtered, detected out-put essentially free ofRF content.

2. The apparatus of claim 1 wherein said 'two tube sections constitutesections of a rmulti-unit tube.

References Cited in the file of this patent UNITED STATES PATENTS MackeyNov. 27, 1956 OTHER REFERENCES Ingram: Spectroscopy .at Radio "andMicrowave Frequencies, Butterworths Scientific Publications, London,1955, pages 102, 103, 235, 236 and 237-.

Blume: The Review ofScientific Instruments, vol. 29,

No. 7, July 1958, pages 574 through 1577.

1. AN OSCILLATOR-LIMITER-DETECTOR COMPRISING A FIRST ELECTRON TUBESECTION SERVING AS A CATHODE FOLLOWER; A TANK CIRCUIT CONNECTED BETWEENGROUND AND THE GRID OF SAID CATHODE FOLLOWER TO PROVIDE THE INPUT TOSAID CATHODE FOLLOWER; A SECOND ELECTRON TUBE SECTION OPERATING AS ACATHODE-DRIVEN DETECTOR, IN WHICH DETECTION TAKES PLACE BETWEEN ITS GRIDAND ITS CATHODE, AND AS A PLATE-LIMITED RF AMPLIFIER; MEANSCATHODE-COUPLING THE SIGNAL FROM THE OUTPUT OF THE CATHODE FOLLOWER TOTHE CATHODE OF SAID SECOND TUBE SECTION; MEANS SUPPLYING A VARIABLEDIRECT PLATE SUPPLY VOLTAGE TO THE PLATE CIRCUIT OF SAID AMPLIFIER OFSUCH A MAGNITUDE THAT THE PLATE VOLTAGE OF SAID AMPLIFIER REMAINS ALWAYSAT LESS THAN ONE HALF THE NORMAL OPERATING VOLTAGE FOR SAID AMPLIFIERTUBE SECTION; MEANS CONNECTED BETWEEN SAID INPUT TANK CIRCUIT AND THEPLATE OF SAID AMPLIFIER TUBE SECTION TO FEED BACK RF SIGNAL FROM SAIDPLATE TO SAID INPUT TANK CIRCUIT; AND MEANS CONNECTED BETWEEN GROUND ANDGRID IN SAID AMPLIFIER TUBE SECTION TO PROVIDE A FILTERED, DETECTEDOUTPUT ESSENTIALLY FREE OF RF CONTENT.